1. Field of the Invention
The present invention relates to a fluorophosphate-base laser glass, and more particularly, it relates to an extremely stable fluorophosphate-base laser glass possessing a very low non-linear refractive index coefficient.
2. Description of the Prior Art
Heretofore, a silicate-base laser glass and a phosphate-base laser glass are known as laser glasses. Japanese patent application (OPI) No. 111,916/74 (corresponding to U.S. patent application Ser. No. 446,085 filed Feb. 26, 1974, now abandoned) discloses a silicate-base glass composed of an SiO.sub.2 -Al.sub.2 O.sub.3 -SrO-Li.sub.2 O-Na.sub.2 O system as a laser glass suitable for high speed and repeating oscillation. Further, Japanese patent application No. 613,795/74 discloses a phosphate-base laser glass composed of a P.sub.2 O.sub.5 -La.sub.2 O.sub.3 -Al.sub.2 O.sub.3 -RO system as a laser glass having far higher gain as compared with the conventional silicate-base glass.
Laser glasses for nuclear fusion have recently been required to have a high grain and also a low non-linear refractive index coefficient n.sub.2 (the refractive index coefficient proportional to the square of the intensity of the electric field applied to the laser). Japanese patent application No. 32,696/75, corresponding to U.S. patent application Ser. No. 667,756 filed Mar. 17, 1976, discloses a P.sub.2 O.sub.5 -Al.sub.2 O.sub.3 -La.sub.2 O.sub.3 -Y.sub.2 O.sub.3 -Na.sub.2 O-Li.sub.2 O series laser glass as a laser glass possessing a low non-linear refractive index coefficient and a high output.
However, the above-described phosphate glass still does not have a satisfactory non-linear refractive index coefficient n.sub.2 and laser glass must have a lower n.sub.2.
The refractive index n of a laser in a high electric field is shown by the equation EQU n = n.sub.0 + n.sub.2 E.sup.2
wherein n.sub.2 is the non-linear refractive index coefficient, E is the intensity of the applied electric field and n.sub.0 is the refractive index coefficient in a non-laser light (non-electric) field. If the non-linear refractive index coefficient is high, the refractive index becomes higher when the laser light passes through the glass whereby self-focusing of the laser glass occurs breaking the laser glass, and, as a result, it is difficult to exhibit high output sufficiently.
As a result of extensive investigations in order to obtain a laser glass possessing a lower non-linear refractive index coefficient n.sub.2 than that of the conventional phosphate laser glass and which is stable as a glass itself, it has now been found that in order to obtain such a glass, it is necessary to reduce the phosphate content in the glass to as low as possible and the reduction of the phosphate content can be made by substituting the glass modifiers with fluorides, and the present invention has been achieved.